The Ames (df) and Snell (dw) dwarf mice provide two good models for studying the induction of tissue-specific transcription during embryogenesis. Both mutants lack the three pituitary cell types which produce prolactin, growth hormone, and thyroid stimulating hormone (TSH). The similarity of the phenotypes of these nonallelic mutants suggests that at least two genes are involved in this differentiation pathway. The simultaneous loss of three cell types suggests that they may derive from a common precursor cell. The dw mutation has been shown to result from defects in the Pit-1 transcription factor gene. The identification and characterization of the df mutation is the focus of this proposal. An intersubspecific backcross will be used to screen candidate genes for linkage to the df mutation on chromosome 11 and for fine mapping of the df gene. Genetically linked molecular probes identified in the backcross will be used to generate a long range physical map of the df region and to screen a yeast artificial chromosome (YAC) library of mouse genomic DNA. Transcribed sequences within the YACs will be identified by several approaches including cross-species homology and direct screening of adult pituitary and embryo head cDNA libraries. Cloning the df gene will clarify the basis for the failure of progenitor cells to differentiate into hormone-producing cells and expand our understanding of the basic cellular commitment process. Genes from the long arm of human chromosome 5 have been mapped to mouse chromosomes 11, 18, and 13. Additional human genes localized on 5q will be placed on the mouse map in an effort to obtain markers closer to the df gene and to determine the extent of synteny conservation. Regions of linkage conservation that are detected by these studies will facilitate positional cloning projects in humans and mice, aid in the correlation of mouse mutants with human diseases, and contribute to our understanding of chromosome evolution. In each dwarf mutant, cell specialization may be blocked at a different stage along the pathway, thereby providing a window into two steps of pituitary differentiation. Several genes have recently been shown to be expressed in the developing pituitary gland prior to Pit-1 expression. The expression of these genes will be characterized in df and dw mutant mouse embryos in order to delineate the sequential steps in the commitment process.